The aims of this project were to develop tools to study the renin angiotensin system in the zebrafish, Danio Rerio, with the expectation that this system has great potential to define early developmental events in renal lineages. The first step in these studies has been to clone and characterize the molecular homologs of the renin angiotensin system in the zebrafish Zebrafish libraries from both adult zebrafish kidney and whole embryo libraries were screened for homologous structures using a human cDNA renin probe. Degenerate primer strategies and screening of EST databases with protein motifs were also used as strategies to identify related genes. The zebrafish angiotensinogen was identifed as was a sequence that is highly homologous with mammalian converting enzyme. The only aspartyl protease identified in zebrafish kidney was a sequence, called zCATD, more homologous with cathepsin D than renin. This molecule includes an internal cleavage site and glycosylation site that is found in mammalian renin and not mammalian cathepsin D. The cDNA was cloned into an expression vecotr and the encoded protein was found to have similar enzymatic activity to native zebrafish renin.These studies led to the hypothesis that cathepsin D functions as the renin homolog and is responsible angiotensin generation in the telosts.